1. Field of the Invention
The present invention relates generally to optical waveguides and also to optical devices utilizing the same, both used in optical network system. More specifically, the present invention relates to technology for removing temperature dependence of the waveguide itself thereby rendering it unnecessary to use additional temperature compensating means.
2. Description of the Related Art
It is known in the art to utilize a DWDM (Dense Wavelength Division Multiplexing) so as to increase the capacity of data transmission in an optical fiber network system. In such a system, a wavelength filter plays an important role in splitting or combining light wavelengths. Such a filter however has encountered the difficulties that the passband thereof (viz., the central wavelength) is apt to undesirably shift because of ambient temperature change.
The fluctuation of the wavelength is caused by temperature dependence of optical path length of optical circuitry constituting the filter. More specifically, the temperature dependence of optical filter is affected by the temperature dependence of refractive index of a material of the waveguide (viz., dn/dT: n is refractive index, and T is temperature), and a linear expansion coefficient of the material. Generally, almost all of silica (quartz) glasses and semiconductors, each of which is typically used as a material of optical waveguide, exhibit positive values in terms of temperature coefficients of refractive index and coefficients of linear thermal expansion. Therefore, the temperature dependence of optical path length (viz., dS/dT: S is optical path length) also represents a positive value.
For example, as the temperature change of 1xc2x0 C. occurs in a silica-based optical wavelength, the central wavelength of a filter changes about 0.01 nm, and this wavelength change corresponds to the central frequency change of about 1.3 GHz. Further, in the case of semiconductor-based optical waveguide, the temperature dependence thereof exhibits approximately, ten times compared with the above-mentioned silica-based waveguide.
In order to stabilize the aforesaid temperature characteristics, it is conceivable to add a precise temperature control device, to the waveguide. However, this technique has encountered the several difficulties that the manufacturing cost becomes high, the device being rendered bulky, and the reliability of the device being lowered.
A known approach to overcoming the above-mentioned problems is to cancel the temperature dependency of the device itself. In view of this, a variety of studies of temperature insensitive optical devices have been conducted.
One example of such studies is disclosed in a paper entitled xe2x80x9cA Temperature Insensitive InGaAsP-InP Optical Filterxe2x80x9d by H. Tanabe, et. al., IEEE Photonics Technology Letters, Vol. 8, No. 11, November 1996, pages 1489-1491 (Related Art 1). The filter, disclosed in this related art, is basically a Mach-Zehnder (MZ) interferometer consisting of two waveguides (or arms) that have different lengths and have different values for dn/dT, low and high. That is, the central wavelength of the filter varies depending on the temperature dependency of the optical path length of each of the two waveguides. Therefore, if the temperature coefficient of the difference between the optical path lengths is made identical, then it is possible to reduce the temperature dependency of the filter.
Another example of techniques pertinent to the present invention is disclosed in a paper entitled xe2x80x9cAthermal silica-based arrayed-waveguide grating multiplexerxe2x80x9d by Y. Inoue, et al., Electronics Letters, 6 th November 1997, Vol. 33, No. 23, pages 1945-1947 (Related Art 2). According to this Related Art 2, in view of the fact that the central wavelength of silica-based arrayed-waveguide grating (AWG) is affected by thermal dependency of the difference between the optical wavelengths of the arrayed-waveguides, a silicon adhesive having a negative thermal coefficient is used to form part of the arrayed-waveguide. As a result, with Related Art 2, a temperature insensitive AWG has been realized by solving the above-mentioned thermal dependency.
However, the techniques disclosed in both Related Art 1 and 2 is limited to the structure wherein the temperature dependent of the filter""s central wavelength is affected by the temperature dependence of the difference between the optical path lengths.
Another known technique is disclosed in Japanese Laid-open Patent Application No. 10-246824 (Related Art 3) wherein a filter taking the form of asymmetrical directional coupler is discussed. More specifically, the temperature dependence of the filter""s central wavelength is rendered negligible by equalizing, at a desirable central wavelength, the propagation constants and the thermal coefficients of effective refractive indices of two optical waveguides. However, this related art is undesirably limited to the application to the filter that takes the form of the asymmetrical directional coupler.
On the other hand, there have been proposed techniques for rendering zero or negligible the temperature dependence of the optical path length of the waveguide itself. These techniques will be described hereinafter.
FIG. 1 is a diagram schematically showing an optical waveguide 8 that is provided with a waveguide layer 10 formed on an appropriate substrate. As shown, the waveguide 8 has a path length L and comprises a cladding 14 and a core 16. Designating an equivalent refractive index of the waveguide layer as neq, then the optical path length S is given by
S=neqxc3x97Lxe2x80x83xe2x80x83(1)
The temperature dependence of the optical path length S (dS/dT) is obtained by differentiating equation (1) with respect to T. Although not clearly shown in FIG. 1, the substrate 12 is typically much thicker than that of the waveguide layer 10. Therefore, the coefficient of linear thermal expansion of the waveguide 8, which is represented by (1/L)(dS/dT), can be approximated to the coefficient of linear thermal expansion of the substrate 12 (xcex1sub). As a result, the temperature dependence of the optical path length, which has been normalized by the waveguide length L, is given by
(1/L)(dS/dT)=(dneq/dT)+neqxc3x97xcex1subxe2x80x83xe2x80x83(2)
In equation (2), the term of the left side (1/L)(dS/dT) is the temperature coefficient of the optical path length, and the first term of the right side (dneq/dT) is the thermal coefficient of the equivalent refractive index. If an optical waveguide, whose temperature coefficient of optical path length (viz., (1/L)(dS/dT)) is zero, is used then it is possible to realize an optical wavelength filter whose central wavelength is rendered independent of temperature.
In other words, in the case where the optical waveguide satisfies the following equation (3), the temperature dependence of the optical path length is rendered zero. Such an optical waveguide is called an athermal waveguide. That is, the athermal condition is given by
(dneq/dT)+neqxc3x97xcex1sub=0xe2x80x83xe2x80x83(3)
In order to satisfy equation (3), it is necessary that the value of (1/L)(dS/dT) is negative while the value of xcex1sub is positive, or vice versa.
One example of an optical filter using an atermal waveguide is disclosed in a paper entitled xe2x80x9cAthermal Narrow-Band Optical Filter at 1.55 xcexcm Wavelength by Silica-Based Athermal Waveguidexe2x80x9d by Y. Kokubun[,] et al., IEICE Trans. Electron., Vol. E81-C, No. 8, August 1998 (Related Art 4). The just-mentioned athermal optical filter is shown in FIG. 2. This athermal filter is provided with a core 20, a lower cladding 22, a substrate 24, an upper loaded cladding 26, and overcladding, 28. This overcladding 28 is made of PMMA (poly-Methyl-Methacrylate) and TFMA (Tri-fluoroethyl-Methacrylate), and accordingly, this known technique has encountered the difficulties because these materials are considerably inferior relative to the crystal materials in terms of heat resistance, moistureproof, strength, aging, etc.
It is therefore an object of the present to provide a temperature insensitive optical waveguide which is able to overcome the difficulties inherent in the known waveguides.
One aspect of the present invention resides in an optical waveguide comprising a substrate and a waveguide layer formed thereon, the waveguide comprising a core and a cladding whose refractive index is less than that of the core, characterized in that the substrate exhibits a negative coefficient of thermal expansion in the temperature range between 0xc2x0 C. and 65xc2x0 C.